Neonatal factors that contribute to obesity are poorly understood. The long-range goal of this research is to clarify how the adipocyte-derived hormone leptin influences development of neuroendocrine pathways that regulate mammalian energy homeostasis. Central to this goal is determining how leptin affects development of neural projections from the arcuate nucleus of the hypothalamus (ARH), a key site for integration of information related to peripheral energy stores. Leptin signals are conveyed via the ARH to other brain regions involved in the regulation of body weight, such as the paraventricular nucleus of the hypothalamus (PVH), an important component of the final common pathway for regulation of energy metabolism. Evidence presented in the body of this proposal supports the concept that leptin functions as a developmental factor during neonatal life by directing formation of neural circuits involved in the control of feeding and energy balance. The overall hypothesis addressed in this proposal is that leptin acts directly on the ARH during a restricted neonatal critical period to promote formation of neural projections to the PVH involved in the regulation of body weight. We propose that the postnatal leptin surge has an enduring effect on ARH projections, and that developmental perturbations in leptin signaling cause permanent changes in neural pathways that transmit leptin signals in mature animals. Both physiological and in vitro experimental approaches will be used to test this hypothesis by addressing the following specific aims. Specific Aim 1. We propose to use axonal labeling methods and histochemical techniques to define the organization and development of ARH projections in obese mice that lack leptin (ob/ob mice) and in diabetic mice that lack a functional long form of its receptor (db/db mice). Specific Aim 2. We will also test the developmental activity of leptin by examining development of the ARH-PVH pathway in ob/ob and db/db mice treated with exogenous leptin, and determine if this developmental activity is restricted to a neonatal critical period. Specific Aim 3. To determine the site of action for leptin in directing development of the ARH-PVH pathway, we will utilize a new explant coculture assay, in addition to examining the direct effects of leptin on axon outgrowth from isolated ARH explants in vitro. Specific Aim 4. In addition, we will examine how nutritional manipulation of leptin levels impacts development of the ARH-PVH pathway. Specific Aim 5. Finally, we will determine if altered patterns of leptin dependent intrahypothalamic signaling accompany observed developmental changes in the ARH-PVH pathway. The results of the proposed research will expand our appreciation of leptin to include a profound developmental activity that promotes formation of leptin-responsive hypothalamic neural pathways. These studies may also provide essential clues about how the neonatal nutritional environment imposes enduring consequences on central regulation of feeding and energy balance throughout life.